I cut a USB cable and used a voltmeter to determine the charging voltage for an iPhone 3GS, iPod 2 and and iPad 2.

The USB port on Apple computers (MacPro and MacBook) put out 5.17 volts. Many of the third-party chargers from the far east (FE) put out 5.00 volts. When charging the iPhone, with the FE chargers I sometimes, but not always, get an error message indicating the the device is not compatible. This doesn't appear to be a problem with the iPod.

Both the Apple and FE iPhone chargers are rated at 1000mA (1 Amp). Apple's iPad chargers are rated at 5.1 V and 2.1 Amps.

Apple's iPhone chargers will charge the iPad, but very slowly because of the lower power rating.